However, your later example is hard for me to follow.
Which (sentence) of the following sentences is considered correct in Australia? (grammatically correct)
Which (sentences) of the following sentences are considered correct in Australia? (also grammatically correct)

No, it wasn't an example or meant to be suggestive of a rule. The first two parts were indeed my opinion. However, the "later example" were two literal questions to which I had been seeking an answer.